The way to look at this, aog, is that had he been prosecuted and convicted in this country he would have received, perhaps, four or five years custody, served about two and would have been released long ago to continue plotting the best way to cause carnage. At least he faces a lengthy period, if not whole life, in custody in the USA and that should provide people some respite from his odious activities for a decent while.
This country (with or without the Human Rights Act) has no stomach to properly prosecute and robustly sentence serious criminals. Take a look at various sentencing guidelines to understand that. Furthermore, it releases those sentenced to determinite periods in prison long before their sentence is served. The authorities are far too busy banning light bulbs and vacuum cleaners to worry about trifling matters such as potential mass murderers being at large.